This invention relates generally to hydraulically operated machines and, in particular, to such machines having multiple tandem articulated members which move in unison in order to produce a desired movement of one of the members. The invention is particularly adapted for use with hydraulically controlled construction equipment, such as hydraulic excavators.
In construction equipment, such as hydraulically operated excavators, one form of control is to provide the operator with manual control of one member such that the member moves in a manner desired by the operator, with other members of the construction equipment automatically controlled, such as by a computer control, in a manner which compensates for the movement of the member manually controlled by the operator. For example, in an excavator having a boom pivotally mounted to a base member, such as an operator's cab, an arm pivotally mounted to the distal end of the boom and a digging bucket pivotally mounted to a distal end of the arm, it is known to provide manual control of the arm and bucket with a control responding to movement of the arm and bucket in order to control movement of the boom in a manner which creates the movement desired by the operator. For example, it is typically desired to excavate on a plane in order to excavate a foundation, pipe trench, or the like. This is accomplished by automatically controlling movement of the boom in a manner which counteracts the variation from a plane which occurs in the movement of the arm and the bucket.
With existing control systems, it is possible for the operator to move the actuator, or joystick, controlling movement of the arm in a manner which creates motion in the arm which is beyond the ability of the control to counteract in controlling the boom. The result is that the excavator excavates in a manner which departs significantly from a plane. In particular, the tendency of the control is to excavate a dip at the start of excavation which is so significantly below the plane of the dig that an unacceptable result is achieved. This result is exaggerated when the arm and bucket are fully, or nearly fully, extended from the base at the beginning of a dig. If the operator fully, or nearly fully, activates the joystick, a velocity is created in the arm and the bucket which cannot be adequately compensated for by the boom under automatic control because the boom's hydraulic valve is incapable of delivering the required hydraulic flow to perform the required counteracting motion. The result of this inadequate boom-countering motion is a further tendency to dig into the ground too deeply initially thereby producing the dip previously discussed.
In U.S. Pat. No. 5,572,809 issued to Timothy E. Steenwyk et al., which is commonly assigned with the present application, a control is provided which compensates for delays accompanying the response of the hydraulic system to commands made upon the system which makes it difficult for the control system to respond in a manner which accurately maintains the cutting edge of the bucket at the desired depth under all conditions. A solution is proposed therein wherein the actuator, or joystick, is monitored in order to provide an input to the control in order to determine anticipated future movement of the arm and thereby compensate for delay between actuation of the arm and counteracting control of the boom. However, it is possible for the operator to actuate the joystick in a manner which exceeds the ability of the control system to compensate with the boom for the resulting actions of the arm.